Expanded mineral particles, such as expanded perlite, are well known. It is also well known that both uncoated and coated, expanded mineral particles such as uncoated and coated, expanded perlite may be used as fillers in various industrial applications.
Perlite, the preferred expanded mineral particle material of the present invention, is in its raw, or pre-expanded state a naturally occurring material of volcanic origin. Raw perlite has a high silica content and contains lesser amounts of alumina, sodium, potassium and very minor amounts of impurities. Pre-expanded perlite ore may have a water content which varies from ore to ore. As is well known, when pre-expanded perlite ore containing water is heated to a range of about 1500.degree. F. to over 2600.degree. F. in conventional expansion furnaces, the perlite particles explosively expand as the contained water flashes to steam and form a distribution of particles which contain a relatively high portion of lightweight, chambered, essentially hollow particles. Such conventionally expanded perlite particles are relatively chemically inert and are also resistant to relatively high temperatures. Such conventionally expanded perlite particles have applications in a wide variety of industrial products such as, for example, horticultural aggregate, insulation and other applications where a lightweight, particulate filler material is useful.
Also, recently, such expanded mineral particles such as expanded perlite, especially coated particles, have been found to be useful as a lightweight filler in pastes, putties, cultured marble, explosives and the like.
Processes and apparatus for expanding mineral products such as perlite are well known. Typically, crushed, dried and sized expandable ore is heated in an open flame and expanded explosively, much like popcorn to provide lightweight, chambered, essentially or hollow, porous particles. Specific methods and apparatus for expansion of naturally occurring perlite ores are well known, with numerous specific methods and devices used to provide improved mineral products or other advantages.
Conventionally expanded particles are known to have uses as fillers, but which uses are limited in many instances because of the high degree of porosity of the expanded product. Because of this relatively high degree of porosity, conventionally expanded perlite is inferior to known non-porous synthetic particles such as glass, ceramic, or plastic hollow microspheres for applications that require durable, lightweight fillers. Such applications include uses such as automobile putties, spackling compounds, caulking formulations and explosives among others.
A well-known and persistent problem associated with conventionally expanded mineral products, such as conventionally expanded perlite particles, is their lack of durability or stability. Within the context of the present invention durability or stability refers to the fact that although they may begin as hollow particles, conventionally expanded mineral particles eventually lose much of their hollowness when used in liquid systems or liquid environments. This eventual, and usually gradual loss of hollowness over time is due to penetration of the host liquid through holes and fissures in the walls of the conventionally expanded mineral particles. The host liquid permeates and essentially fills up the cavities within the conventionally expanded mineral particles. Eventually, the host liquid fills all or essentially all of the voids or cavities in the once hollow, conventionally expanded particles. When the host liquid fills the originally hollow, conventionally expanded particles these particles necessarily lose their lightweight property because their cavities no longer are filled with gases such as air, but rather have become filled or nearly filled with host liquid. When such filling takes place, the density and the viscosity of the liquid and particle mixture, or liquid system increases significantly.
There have been numerous attempts to solve this durability problem. Several attempts to overcome this shortcoming are directed to coating the conventionally expanded mineral particles to provide a barrier to influx of host material.
Although it is known that some benefit has been achieved by coating conventionally expanded mineral particles such as perlite, additional process steps and costs result, and the end product coated particles are still not as effective as known synthetic particles such as glass or plastic microspheres when used in liquid systems where time stability is required. With such coated, expanded mineral products the host liquid nevertheless slowly penetrates into the particle cavities over time to raise the density of the liquid system.
Thus, even coated, conventionally expanded mineral products, such as perlite, have been found to be limited to special applications in commercial use where time stability is generally not required.